1. Field of the Invention
The present invention relates to a Loran-C Receiver Module and more particularly to a module which converts Loran-C RF signals to positional information (latitude/longitude) through the use of a linear receiver and digital signal processing techniques.
2. Description of the Prior Art
Loran is a long-range navigation system in which pulsed signals, received from two or more radio stations, are used by a navigator to determine the geographical position of an airplane or a ship. A standard Loran system consists of a series of radio frequency transmitter stations spaced apart from one another at fixed ground locations. Position is determined by measuring the difference in the time of arrival (TOA) of synchronized, radio frequency pulses transmitted from each of the transmitter stations in the given Loran series or chain. A Loran-C system employs a chain of transmitting stations which includes one master station and two or more secondary stations. The master transmitting station periodically transmits groups of nine Loran pulses. Each secondary transmitting station similarly transmits groups of eight Loran pulses. The above-described Loran pulses are generated by each transmitting station at a group repetition interval (GRI) for that chain. The transmission of pulse groups by the secondary stations is sufficiently delayed in time so as to avoid any overlap in pulse group transmission or reception anywhere in the coverage area of the particular Loran-C chain.
Since the series of pulses transmitted by the master and secondary stations is in the form of pulses of electromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses from a master and secondary station represents the difference in the length of the transmission paths from these stations to the Loran-C receiver.
To determine the location of a receiving station located within the Loran-C coverage area, a Loran-C receiver is synchronized to the group repetition interval (GRI) at which the master and secondary stations of the selected chain transmit at time difference (TD) which is defined as the time of arrival at the receiving station of the secondary station pulse group minus the time of arrival of the master station pulse group. This is determined for each of the secondary stations with respect to the master station. Each of these time differences corresponds to a different line of position (LOP). A line of position is an imaginary line on the surface of the earth exhibiting a constant difference of distance from the master station and a selected station. Thus, from a Loran-C chain having a master station and two secondary stations, two lines of position may be determined. The point at which these two lines of position intersect represents the location of the Loran-C receiver.
When a Loran-C receiver picks up a signal from the master and secondary stations, the signal has already been attenuated by mountains and buildings. Reception of the Loran signal has also been affected by external noise from electric power supply lines. Such influences may cause the signal-to-noise ratio of the Loran signal to drop to 0 db or less. Under these various external conditions, the prior art Loran receivers can not reliably discriminate the Loran pulses from noise.
In order to distinguish the Loran pulses, from the various other signals, a typical Loran system produces sampling pulses synchronized with the Loran pulses. One prior-art Loran receiver having this function is disclosed in Tokkaisho (unexamined published Japanese patent application) 55-2938. It produces a group of sampling pulses at a period of ten microseconds over a group interval of 600 microseconds. In response to each group of sampling pulses the intensity of the received signal is sampled and it is determined whether there is a Loran pulse in the received signal when the sample data exceeds a predetermined level. In more detail, the receiver produces sampling pulses at a period of ten microseconds, each of which includes a pair of pulses separated by 2.5 microseconds. The separation of 2.5 microseconds corresponds to a one quarter period of the carrier, so that one or the other of the pair of sampling pulses will substantially coincide with the peak value of the carrier.
If no Loran pulses are sensed within the interval of 600 microseconds, the timing of the occurrence of the sampling pulse group is shifted by 600 microseconds and sampling is repeated over a new interval of 600 microseconds. If again no Loran pulses are sensed after the shift, the sampling pulse group is again shifted by 600 microseconds until a Loran pulse is detected. This method of sensing Loran pulses is repeated for each of the master and secondary station pulse trains. In order to accurately sense the Loran pulses with this prior art receiver, the sampling pulse group covering 600 microseconds must be repeatedly shifted across a repetition period of 99.7 milliseconds. A relatively long time of about 16.6 seconds is required for that purpose.
In order to attenuate a high noise environment's undesired, interfering signal it is necessary to determine the frequency of the interfering signal prior to or simultaneously with the attenuating signal. One conventional scheme for determining the frequency of an interferer and simultaneously attenuating the same employs a tunable notch filter in the front end of a Loran-C receiver. A metering circuit within the receiver permits the receiver operator to observe the amplitude of the interfering signal. The operator manually tunes the pass band of the notch filter until the interference as indicated on a meter type readout reaches a minimum.
Another conventional approach for determining the frequency of an interfering signal within the Loran-C signal band width employs a voltage controlled oscillator phase locked to the undesired signal. The frequency and phase of the interfering signal are thus automatically tracked without operator intervention.
In a good operating environment a Loran-C receiver can tell the position of the receiver within about 50 feet, but in a poor operating environment the level of accuracy is reduced to about 2500 feet. This increased inaccuracy has long been a problem in the prior art.